How can I disable the termination date field for employees?

You can do this using page composer. You need to make the Termination Date field on the Submit Resignation page read-only for employees and editable for approvers and other users accessing the page.

To do this, you use this EL expression in the page composer:

#{pageFlowScope.resignTerminateFlowMap.sameUserLoggedInFlag == 'Y'}

For more information about using expression language, see Using Expression Language in Oracle Fusion Applications (Doc ID 1982300.1) on My Oracle Support at https://support.oracle.com.